Hanford Hydrogeology
Information about the geology and hydrology of the Hanford Site is used in determining designs and locations of the monitoring wells and provides the basis for evaluation of groundwater flow and contaminant-plume migration.
Structural Setting
The figure shows a generalized cutaway (cross-section) view of the layered sediments under the Hanford Site. The water table is shown by a dashed line and separates the saturated aquifer below from the vadose zone above. Click for a larger image.
This figure shows the stratigraphic units underlying the Hanford Site. The geologic and hydrogeologic columns in the figure show differences in stratigraphy. The geologic column on the right defines the lithostratigraphic units, based on mapping and physical properties of the sediments, modified from BHI-00184. The hydrogeologic column on the left defines hydrostratigraphic units based on hydraulic properties (PNL-8971). Click for a larger image.
The Hanford Site lies in the Columbia Plateau, a broad plain situated between the Cascade Range to the west and the Rocky Mountains to the east. The Columbia Plateau was formed by a thick sequence of volcanic basalt flows, called the Columbia River Basalt Group, that erupted from fissures in north-central and northeastern Oregon, eastern Washington, and western Idaho. Over time, these basalt formations have been folded and faulted, creating a series of basins and ridges. The basalt is exposed on the ridges, and the basins have been subsequently filled by thick sedimentary deposits. The Hanford Site lies within one of the larger basins, known as the Pasco Basin.
Hydrostratigraphy
The sediments in the Pasco Basin (up to hundreds of meters thick) are the primary groundwater-bearing formations in the area and are the principal focus of groundwater monitoring and cleanup efforts. The deeper sediments, in which the pore space is completely filled by groundwater, is known as the "saturated zone" or "uppermost aquifer;" the layer of sediments above the saturated zone, in which the pore space is only partly filled with water and largely filled with air, is known as the "unsaturated zone" or "vadose zone."
The uppermost aquifer beneath most of the Hanford Site is unconfined and is composed of unconsolidated to semiconsolidated sediment of the Hanford and Ringold formations, which were deposited on the basalt bedrock. In some areas, deeper parts of the supra basalt aquifer system are locally confined by layers of silt and clay. Confined aquifers occur within the underlying basalt and associated sedimentary interbeds.
Most of the saturated aquifer exists in the Ringold Formation, which was deposited by the ancestral Columbia and Snake Rivers and consists of sands and gravels with some muddy layers. Above the Ringold Formation is the Hanford formation, which was deposited by cataclysmic floods during past periods of colder climate (ice ages). These floods occurred when large dams formed by ice lobes broke, releasing massive amounts of water from Lake Missoula, a large glacial lake that formed in the Clark Fork River valley in what is today Montana. These floods deposited sediments at Hanford ranging from coarse gravels to fine-grained sands and silts. A third major unit, smaller in volume and extent than the Ringold and Hanford units, is known as the Plio-Pleistocene Unit. This unit consists largely of buried ancient soil horizons, and includes significant amounts of "caliche," a zone of soil that is cemented by calcium carbonate deposits commonly formed in desert environments. Caliche can be very hard and impermeable, and the caliche and fine-grained portions of the Plio-Pleistocene Unit form low-permeability layers that strongly affect movement of water through the vadose zone where they exist.